1. Field of the Invention
The present invention relates to a piezoelectric resonator used in various resonators, oscillators, and similar devices and, more particularly, to a thickness extensional vibration mode piezoelectric resonator constructed to maximize use of harmonics of a thickness extensional vibration mode.
2. Description of the Related Art
Piezoelectric resonators are used in various piezoelectric resonator components such as piezoelectric oscillators and piezoelectric filters. Known piezoelectric resonators of this kind utilize various piezoelectric vibration modes, depending on the frequency desired.
An energy-trap piezoelectric resonator utilizing the second-order wave of a thickness extensional vibration mode is disclosed in Japanese Unexamined Patent Publicaticn No. 117409/1989. This piezoelectric resonator is now described with reference to FIGS. 21 and 22.
The piezoelectric resonator shown in FIGS. 21 and 22 is constructed by stacking ceramic green sheets 51, 52 made of a piezoelectric material on top of each other and sintering the sheets 51, 52 together, as shown in the exploded perspective view of FIG. 21. A circular excitation electrode 53 is disposed in the center of the ceramic green sheet 51. The excitation electrode 53 is extended to an end of the ceramic green sheet 51 via an extraction electrode 54. A circular excitation electrode 55 is disposed in the center of the top surface of the ceramic green sheet 52. The excitation electrode 55 is extended to an end of the ceramic green sheet 52 via an extraction electrode 56. As shown in the lower projected view of FIG. 21, an excitation electrode 57 is disposed on the bottom surface of the ceramic green sheet 52. The excitation electrode 57 is extended to an end of the ceramic green sheet 52 via an extraction electrode 58. It is noted that the electrodes 53, 55, 57 are only partially formed and only partially cover the respective surfaces of the green sheets 51, 52, respectively at a central portion thereof and do not extend across an entire width or length of the sheets 51, 52. That is, the circular electrodes 53, 55, 57 are surrounded in all directions by the surfaces of the respective green sheets 51, 52.
The ceramic green sheets 51 and 52 are stacked on top of each other and pressure is applied in the direction of thickness thereof. Then, the sheets 51, 52 are sintered, thus producing a sintered body. The sintered body is then polarized. Thus, a piezoelectric resonator 60 is obtained, as shown in FIG. 22.
In the piezoelectric resonator 60, piezoelectric layers 61 and 62 are polarized uniformly in the direction of the arrows, i.e., in the direction of thickness.
When the device shown in FIG. 22 is driven, the excitation electrodes 53 and 57 are connected together, and an AC voltage is applied between the excitation electrodes 53, 57 and the excitation electrode 55. In this way, he piezoelectric resonator 60 is driven to resonate such -that the vibration energy is confined to a region where the excitation electrodes 53, 55, 57 overlap each other, i.e., a resonating portion A.
The prior art piezoelectric resonator 60 which is constructed to use the harmonics of a thickness extensional vibration mode is designed as an energy-trap piezoelectric resonator as mentioned above. Therefore, in order to function as an energy trap type resonator, this resonator 60 requires vibration-attenuating portions which are located so as to surround the resonating portion A in all directions for attenuating vibrations created therein. More specifically, because the circular electrodes 53, 55 and 57 are surrounded by surfaces of the respective green sheets 51, 52 and 52 at which vibration-attenuating portions are located, the vibration-attenuating portions have a large size compared with the size of the resonating portion. The large size and arrangement of vibration-attenuating portions in all directions around the electrodes 53, 55, 57 and resonating portion A are necessary to sufficiently suppress vibrations. Thus, because large vibration-attenuating portions are required to suppress vibrations, it has been difficult to reduce the size of the piezoelectric resonator 60.
On the other hand, Japanese Unexamined Patent Publication No. 235422/1990 discloses an energy-trap piezoelectric resonator that uses a piezoelectric ceramic strip and hardly needs extra piezoelectric substrate portions surrounding the resonating portion to attenuate vibrations.
In this device shown in FIG. 23, an excitation electrode 72a and an excitation electrode 72b are located on the top and bottom major surfaces, respectively, of an elongated piezoelectric substrate 71. The excitation electrodes 72a and 72b extend along the entire width and part of the length of the piezoelectric substrate 71, and are disposed opposite to each other with the piezoelectric substrate 71 located therebetween. The electrodes 72a, 72b overlap each other at an approximately central portion of the piezoelectric substrate 71 to define a resonating portion. The excitation electrodes 72a and 72b extend to longitudinal ends 71a and 71b, respectively, of the piezoelectric substrate 71.
When the piezoelectric resonator 70 is excited into a thickness extensional vibration mode, unwanted vibrations occur due to the dimensional relation between the width W and the thickness T of the piezoelectric substrate 71. Accordingly, Japanese Unexamined Patent Publication No. 235422/1990 discloses that where the fundamental wave is used, W/T=5.33 should be used if the resonance frequency is 16 MHz, and that where the third-order wave is used, setting W/T to approximately 2.87 (where the resonance frequency is approximately 16 MHz) can reduce unwanted spurious waves between resonant and antiresonant frequencies.
As described above, the energy-trap piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 117409/1989 and utilizing the second-order wave of a thickness extensional vibration mode needs large vibration-attenuating portions adjacent to the resonating portion. Hence, it is difficult to reduce the size of the resonator.
The energy-trap piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 235422/1990 does not require vibration-attenuating portions adjacent to the resonator portion and so a reduction in size can be attained. However, because harmonic waves of a thickness extensional vibration mode are utilized in this resonator, various unwanted spurious waves appear, in addition to the spurious waves between the resonant and antiresonant frequencies. Because this resonator does not have extra portions surrounding the resonating portion, the spurious waves are generated and are not suppressed. As a result, effective and sufficient resonant characteristics can not be achieved in this resonator.
The piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 235422/1990 has a relatively small electric capacitance and thus, is susceptible to the effects of stray capacitance of the circuit board or the like.